In light sources of the above kind the radiation is emitted from a hot plasma produced by a pulsed current. Very powerful EUV radiation generating devices are operated with metal vapor to generate the required plasma. An example of such a device is shown in WO 2005/025280 A2. In this known EUV radiation generating device the metal vapor is produced from a metal melt which is applied to a surface in the discharge space and at least partially evaporated by a pulsed energy beam, in particular a laser beam. In a preferred embodiment of this device the two electrodes are rotatably mounted forming electrode wheels which are rotated during operation of the device. The electrode wheels dip during rotation into containers with the metal melt. A pulsed laser beam is directed directly to the surface of one of the electrodes in order to generate the metal vapor from the applied metal melt. This evaporation leads to a short circuit between the two electrodes which are connected to a charged capacitor bank, thus igniting the electrical discharge. The resulting current heats the metal vapor such that the desired ionization stages are excited and radiation of the desired wavelength is emitted from a pinch plasma.
With such a technique for generating EUV radiation spatial fluctuations of the discharge region may occur which are not negligible due to the small discharge volume of the pinch plasma. Furthermore, the geometrical form of the EUV or soft x-rays emitting volume normally is not adapted to the optical system using this EUV radiation or soft x-rays, which often comprises circular apertures for guiding the EUV radiation to the reticle and the wafer in case of EUV lithography, for example. Therefore, in such applications the EUV radiation or soft x-rays may not be used effectively.